Twelve weeks at 2,000 mg did not improve insulin sensitivity
Forty obese, insulin-resistant men took 2,000 mg a day for twelve weeks. Insulin sensitivity, glucose handling, liver fat and muscle mitochondria were unchanged.
What the study found
Two grams a day moved nothing, the mitochondria included
Forty obese, insulin-resistant men took 2,000 mg of nicotinamide riboside daily for twelve weeks, measured by clamp and MRI, not blood proxies. Insulin sensitivity, glucose handling, fat mobilisation, liver fat and body composition were unchanged. So were the mitochondria in their muscle.
The caveat · Target molecule never measured
Men only, all obese and insulin-resistant, so this does not describe healthy or older users, and twelve weeks is not years. The mitochondrial half is a sub-study of the same forty men rather than an independent replication, and the two count once. The trial did not report whether NAD actually rose in these participants, so a failure of delivery cannot be fully separated from a failure of the mechanism.